Aqueous particle-free inkjet ink composition

ABSTRACT

An aqueous particle-free inkjet ink composition has a pH of 7 to 9, and consists essentially of: (A) a water-soluble dye colorant present in an amount of 2-10 weight %, (B) a water-soluble polymer present in an amount of 1-4 weight %, and an aqueous medium consisting essentially of water. The water-soluble polymer has an acid number or at least 130 and a molecular weight of 5,000 to 20,000. It comprises: (a) 55-80 mol % of recurring units derived from one or more ethylenically unsaturated polymerizable monomers selected from the group consisting of substituted or unsubstituted styrenes, (meth)acrylates, and combinations thereof, each (meth)acrylate comprising either a pendant phenyl group or an alkyl ester group having 12 to 20 carbon atoms; and (b) 20-35 mol % of recurring units comprising (meth)acrylic acid, all based on total water-soluble polymer recurring units.

RELATED APPLICATION

Copending and commonly assigned U.S. Ser. No. 15/088,181 entitled“Method of Aqueous Inkjet Ink Printing” filed on Apr. 1, 2016, by WayneCook, now allowed.

FIELD OF THE INVENTION

This invention relates to an aqueous particle-free inkjet inkcomposition that can be used to provide images that exhibit improvedoptical print density. The aqueous particle-free inkjet ink compositionconsists essentially of an aqueous medium, a water-soluble dye colorant,and a water-soluble polymer having a unique combination of recurringunits, acid number, and molecular weight.

BACKGROUND OF THE INVENTION

Ink jet printing is a non-impact method for producing printed images bythe deposition of ink drops in a pixel-by-pixel manner onto animage-recording receiver element in response to digital signals. Forexample, inkjet printing can be used for printing a colored image onto areceiver element using a stream of ink droplets directed from an inkjetprinting device.

There are various methods that can be used to control the deposition ofink drops on a receiver element (substrate) to yield the desired inkjetprinted image. Inkjet printing systems are generally of two types:drop-on-demand (DOD) printing systems and continuous ink jet (CIJ)printing systems. For CIJ printing systems, the direction of the streamof ink droplets is controlled electronically and causes the ink dropletsto print the desired image or information on the receiver elementwithout requiring contact between the printing device and the surface towhich the ink is applied. Inkjet printers have found broad applicationsacross markets ranging from desktop document and photographic-quality,to short run printing and industrial labeling. Objects comprisingsubstrates to which inkjet printing is well suited include but are notlimited to, containers for consumer products, currency, draft checks,envelopes, letterhead, identification cards, lottery tickets, bankcards, identification strips, labels, brochures, signage, and otherwell-known materials.

Drop-on-demand printing systems are widely used in home or consumer inkjet printers and slower consumer printers, both of which have beenavailable for several decades. As the name implies, this type of inkjetprinting uses a print head that ejects droplets of ink only whensignaled to do so by a digital controller.

CIJ printing systems generally comprise two main components, a fluidsystem (including an ink reservoir) and one or more print heads. Ink canbe pumped through a supply line from the ink reservoir to a manifoldthat distributes the ink to a plurality of orifices, typically arrangedin linear array(s), under sufficient pressure to cause ink streams toissue from the orifices of the print head(s). Stimulations can beapplied to the print head(s) to cause those ink streams to form streamsof uniformly sized and spaced drop, which are deflected in a suitablemanner, into printing or non-printing paths. Non-printing drops can bereturned to the ink reservoir using a drop catcher and a return line.Thus, in contrast to DOD printing systems, CIJ printing systems involvesuse of a continuous stream of ink drops that are separated todiscriminate between printing drops and non-printing drops. Thisdiscrimination can be accomplished in various ways known in the art.Some useful CIJ printing apparatus and print head fabrication aredescribed for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) andU.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).

Commercially available CIJ inks are commonly aqueous dye-based inks thatexhibit a number of problems. In such dye-based inks, no particles areobservable under the microscope. Although there have been many recentadvances in the art of dye-based ink jet inks, such inks still sufferfrom deficiencies such as low optical densities on coated glossy paperand poor light-fastness. When water is used as the carrier, such aqueousinks also generally suffer from poor water fastness, poor smearresistance, and uncontrolled gloss.

Most aqueous ink jet ink compositions contain non-polymericwater-miscible compounds known as “humectants” to prevent the ink jetinks from drying out in ink jet nozzles during ink jetting. Such dryingout can lead to ink droplet mis-direction and printed image defects. Formost aqueous ink jet ink compositions, such humectants are generallynon-polymeric, nonionic hydrophilic polyols such as glycols, glycolethers and other compounds with one or more hydroxy groups.Representative humectants are described for this purpose, for example,in U.S. Patent Application Publications 2014/0231674 (Cook) and2014/0295152 (Brost et al.) and U.S. Pat. No. 8,142,558 (Robertson etal.). Silicon-containing polymers can be included in aqueous ink jet inkcompositions to reduce wet-smear according to U.S. Pat. No. 6,124,376(Nichols et al.). In addition, UV-curable humectants are described inU.S. Pat. No. 6,846,851 (Nakhmanovich et al.).

It has been observed that images on various papers provided usingaqueous-based inkjet ink formulations containing water-soluble dyestypically exhibit lower print optical density compared to similar imagesprepared using offset lithographic, flexographic, or gravure printing.This is due in part to the penetration of the aqueous-based inkjet inkinto the paper substrate. This problem can be more prominent using CIJprinting inks which generally have lower viscosities than printing inksused in DOD printing systems.

Thus, there is a need for aqueous particle-free inkjet ink compositionsand methods for using them to provide durable and dye image-stableimages having improved print optical density on various types ofreceiver elements (substrates).

SUMMARY OF THE INVENTION

The present invention provides an aqueous particle-free inkjet inkcomposition that can be used to provide inkjet printed images havingimproved print optical density, which aqueous particle-free inkjet inkcomposition has a pH of at least 7 and up to and including 9, andconsists essentially of:

-   -   (A) a water-soluble dye colorant present in an amount of at        least 2 weight % and up to and including 10 weight %, based on        the total weight of the aqueous particle-free inkjet ink        composition,    -   (B) a water-soluble polymer present in an amount of at least 1        weight % and up to and including 4 weight %, based on the total        weight of the aqueous particle-free inkjet ink composition, and    -   (C) an aqueous medium consisting essentially of water,    -   wherein the water-soluble polymer has an acid number or at least        130 and a molecular weight of at least 5,000 and up to and        including 20,000, and comprises: (a) at least 55 mol % and up to        and including 80 mol % of recurring units derived from one or        more ethylenically unsaturated polymerizable monomers selected        from the group consisting of substituted or unsubstituted        styrenes, (meth)acrylates, and combinations thereof, each        (meth)acrylate comprising either a pendant phenyl group or an        alkyl ester group having 12 to 20 carbon atoms; and (b) at least        20 mol % and up to and including 35 mol % of recurring units        comprising (meth)acrylic acid, all recurring unit amounts being        based on the total recurring units in the water-soluble polymer.

The aqueous particle-free inkjet ink compositions of this inventioncontain water-soluble dye colorants that generally require no dispersantin aqueous media. However, in the absence of conventional humectants,the aqueous particle-free inkjet ink composition provides images onvarious substrates that exhibit improved print optical density. It wasalso observed that the aqueous particle-free inkjet ink compositions donot readily dry out in inkjet nozzles.

These advantages are provided by the presence of the water-solublepolymers described herein having a unique combination of compositionalrecurring units, acid number, and molecular weight. Such water-solublepolymers can be used in place of the conventional humectants (especiallythose containing hydroxy groups) yet they appear to keep thewater-soluble colorant (dye) “solubilized” so that the ink does not dryout in the inkjet printing nozzles.

It was unexpectedly found that the printed images obtained using thepresent invention exhibit improved print optical density as determinedusing a reflection spectrophotometer as described in more detail below.In particular, when using the aqueous particle-free inkjet inkcompositions of this invention, the resulting images have a printoptical density that is at least 10%, or at least 15%, or even at least20%, greater than the print optical density achieved under the samecircumstances (for example, conditions including the same substrate)using a “comparative” composition that has the same composition as theinventive composition but from which the inventive water-solublepolymers have been omitted.

DETAILED DESCRIPTION OF THE INVENTION

The following discussion is directed to various embodiments of thepresent invention and while some embodiments can be desirable forspecific uses, for example providing inkjet printed images, thedisclosed embodiments should not be interpreted or otherwise consideredbe limit the scope of the present invention, as claimed below. Inaddition, one skilled in the art will understand that the followingdisclosure has broader application than is explicitly described and thediscussion of any embodiment.

Definitions

As used herein to define various components of the aqueous particle-freeinkjet ink compositions including the water-soluble polymers andwater-soluble dye colorant unless otherwise indicated, the singularforms “a,” “an,” and “the” are intended to include one or more of thecomponents (that is, including plurality referents).

Each term that is not explicitly defined in the present application isto be understood to have a meaning that is commonly accepted by thoseskilled in the art. If the construction of a term would render itmeaningless or essentially meaningless in its context, the termdefinition should be taken from a standard dictionary.

The use of numerical values in the various ranges specified herein,unless otherwise expressly indicated otherwise, are considered to beapproximations as though the minimum and maximum values within thestated ranges were both preceded by the word “about.” In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as the values within the ranges.In addition, the disclosure of these ranges is intended as a continuousrange including every value between the minimum and maximum values.

Unless otherwise indicated, the terms “aqueous particle-free inkjet inkcomposition,” “inkjet ink composition,” and “inventive ink” are meant torefer to embodiments of the present invention.

As used herein, the parameter “acid number” (also known as acid value)is defined as the milligrams (mg) of potassium hydroxide required toneutralize 1 g of the acidic polymer described herein.

The term “particle-free” refers to aqueous inkjet ink compositions ofthe present invention that contain substantially no organic or inorganicparticles such as pigment colorants, colorless inorganic fillers, orpolymeric particles. More particularly, such particles are completelyabsent from such compositions, or at the most, they are present in anamount of less than 0.01 weight %, based on the total weight of theaqueous particle-free inkjet ink composition.

Unless otherwise noted herein, print optical density of an image (inkjetted or otherwise applied to a substrate) was measured using areflectance spectrophotometer in Status T mode using the instructionsthat are provided with the specific instrument used for the evaluation.For example, such evaluations can be carried out using an X-Rite eXact®reflectance spectrophotometer in the Status T mode and operationalconditions defined for that instrument.

Molecular weight for the water-soluble polymers described herein refersto weight average molecular weight (M_(w)) that can be determined usingSize Exclusion (or Gel Permeation) Chromatography calibrated withpolystyrene standards, providing polystyrene equivalent molecularweight.

In reference to the water-soluble polymers and water-soluble dyecolorants used in the present invention, the term “water-soluble” refersto organic compounds that can be dissolved in water to form ahomogeneous composition such that there is no observable settling inwater after 24 hours at room temperature.

Aqueous Particle-free InkJet Ink Compositions

The aqueous particle-free inkjet ink compositions of the presentinvention have only a three essential components to provide the desiredperformance and properties in the resulting printed images. Thus, eachaqueous particle-free inkjet ink composition consists essentially of:(A) one or more water-soluble dye colorants (described below); (B) oneor more water-soluble polymers (described below); and (C) an aqueousmedium consisting essentially of water. There are no components otherthan (A), (B), and (C) that are necessary for the usefulness and desiredproperties of the aqueous particle-free inkjet ink compositions.

Each aqueous particle-free inkjet ink composition generally has asuitable viscosity for use in an inkjet printing machine and especiallyfor CIJ ink jet printing processes and equipment. At 25° C., theviscosity of each aqueous particle-free inkjet ink composition isgenerally less than 10 centipoise or more likely from 1 centipoise andup to and including 5 centipoise, or even at least 1 centipoise and upto and including 3 centipoise. Viscosity can be measured using astandard viscometer under standard conditions.

The pH of each aqueous particle-free inkjet ink composition is generallyat least 7 and up to and including 11 or more likely at least 7 and upto and including 9, or still more likely of at least 7.5 and up to andincluding 8.5. The pH can be obtained and adjusted by use of suitableacids and bases as is known in the art, for example see [0083] of U.S.Patent Application Publication 2014/0231674 (Cook), the disclosure ofwhich is incorporated herein by reference. Measurement of pH can becarried out using standard equipment and procedures.

(A) Water-Soluble Dye Colorants:

The dye colorants used in the present invention are water-soluble asdefined above.

Useful water-soluble dye colorants of this type can be chosen to provideany desired color or hue. They can be used singly or in mixtures toprovide a desired color or hue that can be black, cyan, magenta, yellow,green, brown, orange, pink, blue, red, or violet, or any hue that is notgenerally classified as one of those colors. Such compounds can beanionic dyes, cationic dyes, nonionic dyes, or zwitterionic dyes.

Organic compounds that can be used as water-soluble dye colorantsinclude but are not limited to, Food Black 1, Food Black 2, Food Black40, Carta Black, Direct Black dyes (4, 14, 17, 22, 27, 38, 51, 112, 117,154, and 168), carboxylated Food Black 286, Acid Black dyes (1, 7, 9,24, 26, 48, 52, 58, 60, 61, 63, 92, 107, 109, 118, 119, 131, 140, 155,156, 172, and 194), Acid Red dyes (1, 8, 32, 35, 37, 52, 57, 92, 115,119, 154, 249, 254, and 256), Food Red 40, Direct Red dyes (1, 2, 16,23, 24, 28, 39, 62, 72, 227, and 236), Direct Red 227, Food Yellow 7,Acid Yellow dyes (3, 7, 17, 19, 23, 25, 29, 38, 42, 49, 59, 61, 72, 73,114, 128, and 151), Direct Yellow dyes (4, 11, 12, 27, 28, 33, 34, 39,50, 58, 86, 100, 106, 107, 118, 127, 132, 142, and 157), Food Blue 1,Acid Blue dyes (1, 7, 9, 25, 40, 45, 62, 78, 80, 92, 102, 104, 113, 117,127, 158, 175, 183, 185, 193, and 209), Direct Blue dyes (1, 6, 8, 14,15, 25, 71, 76, 78, 80, 86, 90, 106, 108, 123, 163, 165, 199, and 226),Direct Blue 199, FD&C Blue 1, Acid Orange 7, and others that are knownin the art and available from various commercial sources, includingthose listed in Col. 9 (line 8) to Col. 10 (line 5) of U.S. Pat. No.6,124,376 (Nichols et al.) the disclosure of which is incorporatedherein by reference and in Col. 2 (line 65) to Col. 3 (line 23) whichdisclosure is also incorporated herein by reference. Any anionic dyescan be supplied with suitable counterion(s) such as sodium, lithium,quaternary ammonium, or triethanol amine counterions.

The particularly useful water-soluble dye colorants are Direct Blackdyes, Acid Black dyes, a mixture of two or more Direct Black dyes, amixture of two or more Acid Black dyes, or a mixture of at least oneDirect Black dye and at least one Acid Black dye. Specific examples ofuseful water-soluble black colorants are described in the Examplesbelow.

The one or more water-soluble dye colorants are generally present in theaqueous particle-free inkjet ink composition in an amount of at least 2weight % and up to and including 10 weight %, or at least 4 weight % andup to and including 8 weight %, all based on the total weight of theaqueous particle-free inkjet ink composition (including the aqueousmedium).

(B) Water-Soluble Polymers:

One or more water-soluble polymers are present in the aqueousparticle-free inkjet ink compositions. Each of such water-solublepolymers has an acid number of at least 130 and more likely at least 130and up to and including 240, or even at least 140 and up to andincluding 220.

In addition, each of such water-soluble polymers has a molecular weight(M_(w)) of at least 5,000 or more likely at least 5,000 and up to andincluding 20,000, or even at least 5,000 and up to and including 15,000,or typically at least 6,500 and up to and including 12,000.

Each of the water-soluble polymers used in this invention comprises: (a)at least 50 mol % and up to and including 85 mol % of recurring unitsderived from one or more ethylenically unsaturated polymerizablemonomers selected from the group consisting of substituted orunsubstituted styrenes, (meth)acrylates, and combinations thereof, each(meth)acrylate comprising either a pendant phenyl group or an alkylester group having 10 to 20 carbon atoms; and (b) at least 15 mol % andup to and including 40 mol % of recurring units comprising a carboxylicacid group, all recurring unit amounts being based on the totalrecurring units in the water-soluble polymer.

In some embodiments, each of such water-soluble polymers comprises: (a)at least 55 mol % and up to and including 80 mol % of recurring unitsderived from one or more ethylenically unsaturated polymerizablemonomers selected from the group consisting of substituted orunsubstituted styrenes, (meth)acrylates, and combinations thereof, each(meth)acrylate comprising either a pendant phenyl group or an alkylester group having 12 to 18 carbon atoms; and (b) at least 20 mol % andup to and including 35 mol % of recurring units comprising a carboxylicacid group, all recurring unit amounts being based on the totalrecurring units in the water-soluble polymer.

In more specific embodiments, each of the water-soluble polymerscomprises: (a) at least 55 mol % and up to and including 80 mol % ofrecurring units derived from one or more ethylenically unsaturatedpolymerizable monomers selected from benzyl (meth)acrylates, asubstituted or unsubstituted styrene, and a combination thereof; and (b)at least 20 mol % and up to and including 35 mol % of recurring unitsderived from (meth)acrylic acid, all recurring unit amounts being basedon the total recurring units in the water-soluble polymer.

The (a) recurring units described above can be derived from one or moreethylenically unsaturated polymerizable monomers selected fromsubstituted or unsubstituted styrenes such as styrene, α-methyl styrene,p-bromostyrene, o- or m-chlorostyrene or mixtures thereof,dimethoxystyrenes, p-aminostyrene, t-butylstyrene,4-trimethyleneidoxystyrene, and other “monomers” of this type that wouldbe readily apparent to one skilled in the art. Substituted orunsubstituted styrene is particularly useful for providing such (a)recurring units and unsubstituted styrene is more useful.

The (a) recurring units can be also have pendant alkyl ester groups andbe provided from one or more (meth)acrylates each of which compriseseither a pendant substituted or unsubstituted phenyl group or an alkylester group having 10 to 20 carbon atoms (or likely at least 12 to 18carbon atoms). For example, such (a) recurring units can be providedfrom benzyl methacrylate, stearyl methacrylate, lauryl methacrylate,stearyl acrylate, lauryl acrylate, benzyl acrylate, 4-methyl benzylmethacrylate, palmityl acrylate, palmityl methacrylate, and other“monomers” of this type that would be readily apparent to one skilled inthe art. Benzyl methacrylate, stearyl methacrylate, and laurylmethacrylate are particularly useful.

The (a) recurring units can be present in an amount of at least 50 mol%, or typically of at least 50 mol % and up to and including 85 mol %,or at least 55 mol % and up to and including 80 mol %, or even at least55 mol 5 and up to and including 75 mol %, all recurring units amountsbeing based on the total recurring units in the water-soluble polymer.

The (b) recurring units comprising one or more carboxylic acid groupsdescribed above can be derived from acrylic acid, methacrylic acid,crotonic acid, tiglic acid, 4-vinylbenzoic acid, maleic acid, fumaricacid, citaconic acid, methylmaleic acid, and other “monomers” withcarboxylic acid groups that would be readily apparent to one skilled inthe art, as well as precursor ethylenically unsaturated polymerizablemonomers such as corresponding anhydrides or halides.

The (b) recurring units can be present in an amount of at least 15 mol%, or typically of at least 15 mol % and up to and including 40 mol %,or at least 20 mol % and up to and including 35 mol %, all recurringunits amounts being based on the total recurring units in thewater-soluble polymer.

The water-soluble polymers useful in this invention can further compriseadditional recurring units besides those (a) and (b) recurring unitsidentified herein, which additional recurring units can be present in anamount of up to and including 35 mol % or up to and including 20 mol %,based on the total recurring units in the water-soluble polymer. In someembodiments, there are no additional recurring units in thewater-soluble polymers. Some particularly useful additional recurringunits can be derived from one or more of ethoxyethoxyethyl acrylate,ethoxyethoxyethyl methacrylate, and other “monomers” that are notdefined above for making the (a) and (b) recurring units, which could bereadily determined by one skilled in the art using routineexperimentation.

The water-soluble polymers can be used with free carboxylic acid groups,but in many embodiments, up to 100% of the carboxylic acid groups areneutralized with a suitable cation such as potassium, sodium or otheralkali metal cations provided as part of suitable salts or bases, orwith an amine such as dimethylethanol amine, methyldiethanolamine, orsimilar compounds that would be readily apparent to one skilled in theart.

Particularly useful water-soluble polymers include but are not limitedto the following materials (weight ratios refer to relative amounts ofethylenically unsaturated polymerizable monomers incorporated into thereaction mixtures used to prepare the polymers):

-   -   Polymer 1: poly(benzyl methacrylate-co-stearyl        methacrylate-co-methacrylic acid) (35:30:35 weight ratio),        neutralized with potassium hydroxide, for example having an acid        number of 215 before neutralization and M_(w) of 9,000;    -   Polymer 2: poly(benzyl methacrylate-co-stearyl        methacrylate-co-methacrylic acid) (35:30:35 weight ratio),        neutralized with dimethylethanolamine, for example having an        acid number of 215 before neutralization and a M_(w) of 9,000;    -   Polymer 3: poly(styrene-co-α-methyl styrene-co-acrylic        acid-co-ethoxyethoxyethyl acrylate) (52:7:29:12 weight ratio),        neutralized with potassium hydroxide (commercially available as        Joncryl® HPD-696 polymer from BASF), for example, having an acid        number of 220 before neutralization and a M_(w) of 16,000;    -   Polymer 4: poly(styrene-co-methacrylic acid) (64:36 weight        ratio), neutralized with dimethylethanolamine, for example        having an acid number of 234 before neutralization and a M_(w)        of 21,700;    -   Polymer 5: poly(lauryl methacrylate-co-methacrylic acid) (67:33        weight ratio), neutralized with dimethylethanolamine, for        example having an acid number of 215 and a M_(w) of 10,800;    -   Polymer 6: poly(benzyl methacrylate-co-methacrylic acid) (67:33        weight ratio), neutralized with potassium hydroxide, for example        having an acid number of 215 and a M_(w) of 8,000;    -   Polymer 7: poly(benzyl methacrylate-co-stearyl        methacrylate-co-methacrylic acid) (63.5:15:21.5 weight ratio),        neutralized with potassium hydroxide, for example, having an        acid number of 140 and a M_(w) of 8,000;    -   Polymer 8: poly(benzyl methacrylate-co-styrene-co-methacrylic        acid) (30:45:25 weight ratio), neutralized with dimethylethanol        amine, for example having an acid number of 163 and a M_(w) of        10,100; and    -   Polymer 9: poly(benzyl methacrylate-co-methacrylic acid) (77:23        weight ratio), neutralized with potassium hydroxide, for example        having an acid number of 150 and an M_(w) of 7,000.

The mol % numbers provided above for the various (a), (b) and (c)recurring units can refer to either actual molar amounts determined fromanalysis or theoretical molar amounts determined from the molar ratiosof the various ethylenically unsaturated polymerizable monomers used inpreparing the water-soluble polymers. In most embodiments, the actualand theoretical mol % amounts differ by less than 15%. The weight % ofvarious useful ethylenically unsaturated polymerizable monomers used inreaction mixtures to prepare the water-soluble polymers can becalculated in order to achieve the desired mol % of recurring units.

The water-soluble polymers useful in the present invention can beprepared using known polymerization starting materials (ethylenicallyunsaturated polymerizable monomers), organic solvents (for example,methyl ethyl ketone or Dowanol® PM commercial solvent), photoinitiators,chain transfer agents (for example, dodecanethiol), and conditions forsolution polymerization. Alternatively, some water-soluble polymers canbe obtained from various commercial sources. After formation of thewater-soluble polymer, it can be neutralized with a suitable base asnoted above, and the reaction solution can be diluted with water. Theorganic reaction solvent can be removed in a suitable manner to isolatethe water-soluble polymer.

The total amount of the one or more water-soluble polymers in theaqueous particle-free inkjet ink composition is at least 1 weight % orlikely at least 1 weight % and up to and including 4 weight %, based onthe total weight of the aqueous particle-free inkjet ink composition.More specifically, the total amount of such polymers can be at least 1.5weight % and up to and including 3 weight %, based on the total weightof the aqueous particle-free inkjet ink composition.

In some embodiments, the aqueous particle-free inkjet ink compositioncomprises a water-soluble polymer in an amount of at least 1.5 weight %and up to and including 3 weight %, and a water-soluble dye colorant inan amount of at least 4 weight % and up to and including 8 weight %, allamounts being based on the total weight of the aqueous particle-freeinkjet ink composition.

(C) Aqueous Medium:

The last essential component is an aqueous medium that consistsessentially of water, meaning that water is at least 90 weight %, or atleast 95 weight %, or even 100 weight % of the total solvents in theaqueous particle-free inkjet ink composition. In addition, the aqueousmedium generally comprises at least 80 weight % or at least 90 weight %but less than 97 weight % of the total weight of the aqueousparticle-free inkjet ink composition. In most embodiments, the aqueousmedium is 100% water.

While not essential to the practice of this invention, the aqueousparticle-free inkjet ink compositions can also include one or moreoptional additives that can enhance formulation, ink jetting properties,shelf life before printing, or glossiness of the ink jet printed image.Such materials can include but are not limited to, to acids or bases toadjust the pH, biocides, surfactants to adjust the static surfacetension or dynamic surface tension of the aqueous ink jet inkcompositions, defoaming agents, pH buffers, corrosion inhibitors,viscosity modifiers, wetting agents, chelating agents, antifungalagents, jetting aids, filament length modifiers, thickeners, dryingagents, flocculating agents, waterfastness agents, dye solubilizers,light stabilizers, anti-curl agents, and other components that would beknown to one skilled in the art, all of which components can be presentin small amounts that would be known in the art for optimal performance.Examples of such materials are known in the art.

However, the aqueous particle-free inkjet ink compositions of thepresent invention do not contain appreciable or effective amounts of“humectants” that are typically used in inkjet ink compositions andgenerally comprise at least one hydroxyl group per molecule. Suchcompounds are generally completely absent or present in an amount ofless than 0.1 weight %, based on the total weight of the aqueousparticle-free inkjet ink composition. Representative humectants aredescribed, for example, in [0074]-[0082] of U.S. Patent ApplicationPublication 2014/0231674 (noted above).

The aqueous particle-free inkjet ink compositions of the presentinvention can be prepared simply by mixing the essential (A) and (B)components in the (C) aqueous medium with suitable mixing, in anydesired order, at suitable temperature and in proportions to achieve thedesired concentrations. Optional additives described above can also beincluded during the formulation process if desired. Typically, themixing can be carried out at room temperature with moderate stirring.Filtering can be used to remove any solids or particulates. Theformulated aqueous particle-free inkjet ink compositions can be usedimmediately upon mixing or stored or shipped for later use by variouscustomers.

Ink Sets

The aqueous particle-free ink jet ink compositions of this invention canbe provided as part of an ink set. Such ink sets can therefore includemultiple aqueous inkjet ink compositions of the same or differentcolors. Each aqueous inkjet ink composition in the ink set (other thanthe aqueous particle-free inkjet ink composition described for theinvention) can be formulated in any particular fashion using anydesirable component and need not be particle-free. The aqueous inkjetink compositions that contain particles can include one or more pigmentcolorants.

For example, a useful ink set can include for example, multiple aqueouscolor ink jet ink compositions chosen from an aqueous cyan ink jet inkcomposition, an aqueous magenta ink jet ink composition, an aqueousyellow ink jet ink composition, and an aqueous particle-free black inkjet ink composition according to the present invention.

Methods of Ink Jet Printing

The present invention can be used for inkjet printing and forming atleast one colored image on a suitable substrate (or receiver element,described below). This is generally accomplished by incorporating one ormore aqueous particle-free inkjet ink compositions of this inventioninto a suitable ink jet printing apparatus containing a main fluidsupply and ink jetting droplets of each of the one or more aqueousparticle-free inkjet ink compositions to be ejected in an imagewisepattern onto the receiver element. In some inkjet systems, the mainfluid supply is a recirculating main fluid supply containing the aqueousparticle-free inkjet ink composition according to the present invention.

For example, an inkjet printed image can be provided by inkjet printingone or more of the aqueous particle-free inkjet ink compositions usingthermal or piezoelectric drop-on-demand (DOD) printheads or continuous(CIJ) printheads, each printhead being incorporated into DOD orcontinuous inkjet printing apparatus, respectively, containing a mainfluid supply, for example a recirculating main fluid supply, containingthe aqueous particle-free inkjet ink composition. Each type of printheadand apparatus attached thereto requires specific properties in theaqueous particle-free ink jet ink compositions in order to achievereliable and accurate jetting of each composition.

For example, the method for inkjet printing can comprise:

-   -   providing a suitable receiver element,    -   providing a main fluid supply (such as a recirculating main        fluid supply) of an aqueous particle-free inkjet ink composition        of the present invention, and    -   pumping the aqueous particle-free inkjet ink composition from        the main fluid supply (such as a recirculating main fluid        supply) to a drop generator mechanism, and    -   ejecting a continuous stream of the aqueous particle-free inkjet        ink composition from the drop generator mechanism, which        continuous stream from the main fluid supply (such as a        recirculating main fluid supply) is broken into printing ink        drops and non-printing ink drops to provide a waterfast inkjet        printed image on the receiver element.

In some embodiments, such method also includes, the printed ink iscontrolled in response to electrical signals received from a controlmechanism, to differentiate between printing ink drops and non-printingink drops, and the non-printing ink drops are collected and returned toa recirculating main fluid supply.

For example, in many embodiments, the aqueous particle-free inkjet inkcomposition of this invention has a dedicated delivery channel to avoidhaving oppositely charged materials in the multiple aqueous inkjet inkcompositions interact until they come into reactive association to forma composite color image on a receiver element. The multiple aqueousinkjet ink compositions can be applied (ink jetted) either in the samepass or in different passes through an ink jet printer such as the CIJapparatus described herein.

When the method is carried out using CIJ apparatus and processes, themethod can include:

-   -   ink jetting the aqueous particle-free inkjet ink compositions        described herein as a continuous stream from a recirculating        main fluid supply, breaking the continuous stream into both        spaced ink drops and non-printing ink drops; and    -   collecting and returning the non-printing ink drops to the        recirculating main fluid supply.

For all of the ink jetting operations and particularly for ink jettingthe aqueous particle-free inkjet ink composition using CIJ methods andequipment, ink jetting can be carried out at a suitable firing frequencyof at least 350 kHz with a near nozzle velocity of at least 15 m/sec.

The aqueous particle-free inkjet ink composition of the presentinvention can be positioned in any one of the printhead ports intendedfor use in an ink jet printing apparatus. The printhead containing theaqueous particle-free ink jet ink composition can be positioned on thesame carriage assembly as another aqueous ink jet ink composition, orthey can be positioned on separate carriage assemblies.

The present invention can be used to provide a suitable colored inkjetprinted image on a suitable receiver element. For example, suitablereceiver elements can include but are not limited to, photoglossyreceivers or plain paper receivers such as bright white inkjet papersthat are commercially available from a number of commercial sources (asdescribed for example, in Col. 13, lines 28-34) of U.S. Pat. No.8,187,371 (Brunt et al.). The photoglossy receivers (also known asswellable media or microporous media) can be manufactured with a coatedlayer on an underlying paper support and are also useful for providingphotographic quality ink jet printed images. Some details of suchsubstrates are provided in Col. 13 (lines 37-51) of U.S. Pat. No.8,187,371 (noted above). Plain papers can be treated with multivalentsalts or cationic polymers during or after manufacture as is well knownin the art. Other useful receiver elements are described in U.S. Pat.No. 6,045,917 (Missell et al.), U.S. Pat. No. 5,605,750 (Romano et al.),U.S. Pat. No. 5,723,211 (Romano et al.), and U.S. Pat. No. 5,789,070(Shaw-Klein et al.).

Useful paper receiver elements include plain bond papers,surface-treated papers, coated or calendared business gloss papers,resin-coated papers, laminated substrates comprising both paper layersand polymeric film layers such as polyester film layers, lottery ticketstock, and heavy stock papers. It is also possible to use fabrics,cardboard, plastic films (such as polyester films), microporousmaterials, and any other material known in the art. Thus, the receiverelement can be transparent, translucent, or opaque.

The durability and other properties of ink jet printed images providedusing this invention can be improved by using receiver elements thathave been pretreated with a composition to enhance the quality of theink jet printed images. This pretreatment is typically done prior toincorporation of the receiver element into the inkjet printing apparatus(such as a continuous ink jet printing apparatus), but in someinstances, the receiver element can be pretreated within the apparatusbefore application of an aqueous particle-free inkjet ink composition.One or both sides of a receiver element can be pretreated, or one sidecan be pretreated and the opposite surface left untreated.

Such pretreatment compositions can also comprise a cationicpolyelectrolyte comprising amidine moieties, and the details of suchcompounds and their useful amounts are provided in U.S. Pat. No.8,562,126 (Xiang et al.), the disclosure of which is incorporated hereinby reference. Other cationic polymers, such as apoly(dialkyldimethylammonium chloride) can also be used.

As noted from the data provided in the Examples below, the improvementin print optical density can vary with the type of substrate used andthe amount and composition of water-soluble polymer used in the aqueousparticle-free inkjet ink composition.

A resulting printed receiver element can be any suitable article, andincludes but is not limited to, documents, paper currency, postagestamps, various packaging materials, fabrics, polymeric films or sheets,label for clothing, perfume and wine bottles labels, gaming articles,passports, drivers licenses, and other articles that would be readilyapparent to one skilled in the art using the teaching provided herein.

It is also practical that the spaced ink drops of the aqueousparticle-free ink jet ink compositions are dried on the receiver elementafter they have been ink jetted.

Some embodiments of the aqueous particle-free inkjet ink compositionsare designed specifically for use in a continuous ink jet printer, inwhich a recirculating main fluid supply is provided with a desiredamount of the aqueous particle-free ink jet ink composition, which isthen pumped from the recirculating main fluid supply to a drop generatormechanism such as one or more ink jet print heads, where a continuousstream of the aqueous particle-free inkjet ink composition is ejectedfrom the drop generator mechanism, which continuous stream then isbroken into spaced ink drops and non-printing ink drops.

Some details of CIJ printing operations and apparatus are described morefully for example in U.S. Pat. No. 4,734,711 (Piatt et al.), U.S. Pat.No. 5,394,177 (McCann et al.), U.S. Pat. No. 6,588,888 (noted above),U.S. Pat. No. 6,943,037 (noted above), U.S. Pat. No. 4,614,948(Katerberg et al.), and U.S. Pat. No. 4,971,527 (Dick), the disclosuresof which are incorporated by reference herein. In contrast todrop-on-demand printing, CIJ is a very high speed printing process andit is designed to operate at substrate transport speeds in excess of 200m/min.

A useful sub-system common to most CIJ apparatus and methods and to someof the more recent DOD printing systems, is a recirculating fluid systemwith a recirculating main fluid supply, which constantly recirculatesthe aqueous particle-free ink jet ink composition.

In some embodiments of the present invention, the aqueous particle-freeinkjet printer ink composition is printed by employing a plurality ofdrop volumes (or drop sizes) formed from the continuous fluid stream,with non-printing ink drops of a different volume than printing dropsbeing diverted by a drop deflection means into a gutter forrecirculation, as disclosed for example in U.S. Pat. No. 6,588,888(noted above), U.S. Pat. No. 6,554,410 (Jeanmaire et al.), U.S. Pat. No.6,682,182 (Jeanmaire et al.), U.S. Pat. No. 6,793,328 (Jeamnaire), U.S.Pat. No. 6,517,197 (Hawkins et al.), U.S. Pat. No. 6,866,370(Jeanmaire), and U.S. Pat. No. 6,575,566 (Jeanmaire et al.), U.S. PatentApplication Publication 2003/0202054 (Jeanmaire et al.), the disclosuresof all of which are incorporated herein by reference.

In other embodiments, the aqueous particle-free inkjet ink compositionis printed onto at a suitable receiver element using an apparatuscapable of controlling the direction of the formed printing ink dropsand non-printing ink drops by asymmetric application of heat to thecontinuous stream of the aqueous particle-free inkjet ink compositionthat initializes drop break-up and serves to steer the resultant drops,as disclosed for example in U.S. Pat. No. 6,079,821 (Chwalek et al.) andU.S. Pat. No. 6,505,921 (Chwalek et al.), the disclosures of both ofwhich are herein incorporated by reference. Useful agitation of theaqueous particle-free inkjet ink composition, heated recirculating mainfluid supply, and ink jet print head and composition filtration meansfor CIJ ink compositions are described for example in U.S. Pat. No.6,817,705 (Crockett et al.), the disclosure of which is incorporatedherein by reference. Printer replenishing systems for maintainingquality of the aqueous particle-free ink jet ink composition and tocounter the effects of volatile component evaporation are described inU.S. Pat. No. 5,526,026 (Bowers) and U.S. Pat. No. 5,473,350 (Mader etal.), the disclosures of which are incorporated herein by reference, andin EP 0 597 628 A1 (Loyd et al.).

It can be useful to regularly replenish the recirculating main fluidsupply with additional amounts of each aqueous particle-free inkjet inkcomposition described herein to keep the reservoir at the desired levelduring ink jet printing. Alternatively, water can be added to therecirculating main fluid supply to compensate for evaporation duringinkjet printing. A skilled worker in the art would understand how toaccomplish these operations using the teaching provided in the art notedabove.

The following Examples are provided to illustrate the practice of thisinvention and are not meant to be limiting in any manner.

The specific water-soluble polymers used in the following Examples aredescribed above along with acid numbers and molecular weights for thespecific materials prepared or purchased for use in the presentinvention.

Aqueous particle-free inkjet ink compositions (“inks”) were preparedaccording to the present invention to contain the following components(while excluding conventional humectants) wherein all amounts are basedon the total composition weight:

-   -   Water 50.03 weight %    -   Triethanolamine 0.5 weight %    -   Proxel™ GXL biocide 0.1 weight %    -   Kordek™ MLX biocide 0.2 weight %    -   Cobratec TT50-S        -   corrosion inhibitor 0.1 weight %    -   Surfynol® 440 surfactant 0.08 weight %    -   Surfynol® DF-110L defoamer 0.1 weight %    -   Sulfuric acid (1 normal) 2.70 weight %    -   Direct Black 168 dye 2.02 weight %        -   (27% dye solids)    -   Direct Black 19 dye 34.17 weight %        -   (17% dye solids)    -   Polymer 9 1.25-3.75 weight % (in 25% water solution)

Each aqueous particle-free inkjet ink composition was formulated bymixing all listed components together, and had a pH of between 8 and 9and viscosity of 2-2.5 centipoise.

Comparative Example 1

A comparative aqueous particle-free inkjet ink composition (“ink”) wasformulated outside the present invention, using the formulationdescribed above except that water-soluble polymers were omitted (polymerweight was replaced with water). The resulting “ink” had a similar pHand viscosity as described above.

Inventive Examples 1-5

The Comparative Example 1 “ink” (0 weight % water-soluble polymers) andvarious aqueous particle-free inkjet ink compositions (“inks”) preparedusing the formulation described above (using 1.25 weight %, 1.875 weight%, 2.5 weight %, 3.125 weight %, and 3.75 weight % of Polymer 9) wereused to provide black images on 25# TrueJet paper substrate (availablefrom Verso Paper Company). A drop of each “ink” was deposited upon asample of the paper substrate, and spread on the surface of thesubstrate sample using a #4 wire wound rod to provide a generallyuniform ink coating, and then dried with heated air. The print opticaldensity of each resulting black image was determined using an X-RiteeXact® reflectance spectrophotometer in the Status T mode. The resultsof these measurements for the various black images are shown in thefollowing TABLE I.

TABLE I Ink Weight % Print Optical Example Polymer 9 Density Comparative1 0 1.49 Inventive 1 1.250 1.52 Inventive 2 1.875 1.60 Inventive 3 2.5001.73 Inventive 4 3.125 1.76 Inventive 5 3.750 1.82

These data show that the addition of Polymer 9 according to the presentinvention provided improved print optical density especially compared tothe ink from which the water-soluble polymer was absent. The improvementincreased as the amount of the Polymer 9 was increased. In thoseembodiments in which the amount of Polymer 9 was at least 2.500 weight%, the print optical density of the black image was at greater than 10%greater than the print optical density for the black image obtained whenPolymer 9 was absent. The point at which the print optical densityimprovement would be at least 10% is likely between 1.875% and 2.500%water-soluble polymer for the particular substrate used in theseexperiments.

Inventive Examples 6-13

Aqueous particle-free inkjet ink compositions were prepared using theformulation described above for Inventive Examples 1-5 and severalwater-soluble polymers according to the present invention. A drop ofeach “ink” was applied to samples of Verso 80# Sterling Ultra Glosspaper substrate and spread as described above in Inventive Examples 1-5,and print optical density values of the resulting black images weresimilarly determined and shown in the following TABLE II.

TABLE II Weight % of Print Ink Water-soluble Water-soluble OpticalExample Polymer Polymer Density* Comparative 1 None 0 1.28 Inventive 1 92.50 1.49 Inventive 7 1 1.70 1.44 Inventive 8 2 1.70 1.38 Inventive 9 32.50 1.43 Inventive 10 4 2.50 1.49 Inventive 11 5 1.88 1.34 Inventive 126 2.50 1.41 Inventive 13 7 2.00 1.47 Inventive 14 8 1.88 1.55 *Valuesnormalized to equal water-soluble polymer content

The data in TABLE II shows that several water-soluble polymers describedaccording to the present invention are useful to provide improved printoptical density compared to the Comparative Example 1 ink from which thewater-soluble polymers were omitted. In most instances, the presence ofthe water-soluble polymer provided an improvement in print opticaldensity of at least 10%, and in some instances, the improvement was atleast 15%, and even at least 20% for one inventive “ink”.

Inventive Examples 14-33

Water-soluble Polymer 9 was incorporated into severalaqueous-particle-free inkjet ink compositions at various % solidsaccording to the present invention using the formulation described abovein Inventive Examples 1-5. Samples of each “ink” were inkjet printedonto samples of each of four paper substrates identified in TABLE IIIbelow using a laboratory model “continuous” inkjet printer in Ohio PrintMode, 600×600 dpi, 400 kHz frequency, and 1000 feet/minute printingspeed.

The print optical density values for each printed black image wasdetermined using an X-Rite eXact® reflectance spectrophotometer in theStatus T mode and the results are shown below in TABLE III.

TABLE III Weight Print Ink % of Paper Optical Example Polymer 9Substrate Density Comparative 1 0 Verso 80# Sterling Ultra 1.128 Glosspaper Inventive 14 2.500 ″ 1.344 Inventive 15 1.250 ″ 1.262 Inventive 161.875 ″ 1.276 Inventive 17 3.125 ″ 1.342 Inventive 18 3.750 ″ 1.404Comparative 1 0 25# TrueJet paper 1.262 Inventive 19 2.500 ″ 1.516Inventive 20 1.250 ″ 1.410 Inventive 21 1.875 ″ 1.458 Inventive 22 3.125″ 1.578 Inventive 23 3.750 ″ 1.626 Comparative 1 0 International Paper24# 1.012 Dataspeed ImageLok ® paper Inventive 24 2.500 ″ 1.178Inventive 25 1.250 ″ 1.156 Inventive 26 1.875 ″ 1.172 Inventive 27 3.125″ 1.170 Inventive 28 3.750 ″ 1.190 Comparative 1 0 Appleton 24# CoatedUtopia 1.108 Book Inkjet paper Inventive 29 2.500 ″ 1.338 Inventive 301.250 ″ 1.264 Inventive 31 1.875 ″ 1.274 Inventive 32 3.125 ″ 1.332Inventive 33 3.750 ″ 1.390

The data in TABLE III show that the present invention can be used atvarying amounts of water-soluble polymer in the aqueous particle-freeinkjet ink compositions to provide improved print optical density forinkjet printed images on various types of commercial substrates.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

The invention claimed is:
 1. An aqueous particle-free inkjet inkcomposition that has a pH of at least 7 and up to and including 9, andconsists essentially of: (A) a water-soluble dye colorant present in anamount of at least 2 weight % and up to and including 10 weight %, basedon the total weight of the aqueous particle-free inkjet ink composition,(B) a water-soluble polymer present in an amount of at least 1 weight %and up to and including 4 weight %, based on the total weight of theaqueous particle-free inkjet ink composition, and (C) an aqueous mediumconsisting essentially of water, wherein the water-soluble polymer hasan acid number of at least 130 and a weight average molecular weight ofat least 5,000 and up to and including 20,000, and comprises: (a) atleast 55 mol % and up to and including 80 mol % of recurring unitsderived from one or more ethylenically unsaturated polymerizablemonomers selected from the group consisting of substituted orunsubstituted styrenes, (meth)acrylates, and combinations thereof, each(meth)acrylate comprising either a pendant phenyl group or an alkylester group having 12 to 20 carbon atoms; and (b) at least 20 mol % andup to and including 35 mol % of recurring units comprising (meth)acrylicacid, all recurring unit amounts being based on the total recurringunits in the water-soluble polymer.
 2. The aqueous particle-free inkjetink composition of claim 1, wherein the water-soluble dye colorant is adirect black dye, an acid black dye, or a blend of two or more of suchdyes.
 3. The aqueous particle-free inkjet ink composition of claim 1,wherein the water-soluble dye colorant is present in an amount of atleast 4 weight % and up to and including 8 weight %, based on the totalweight of the aqueous particle-free inkjet ink composition.
 4. Theaqueous particle-free inkjet ink composition of claim 1, wherein atleast 98 weight % of the aqueous medium consists of water.
 5. Theaqueous particle-free inkjet ink composition of claim 1, wherein thewater-soluble polymer has an acid number of at least 130 and up to andincluding 240 and a weight average molecular weight of at least 5,000and up to and including 15,000.
 6. The aqueous particle-free inkjet inkcomposition of claim 5 wherein the water-soluble polymer comprises: (a)at least 55 mol % and up to and including 80 mol % of recurring unitsderived from one or more ethylenically unsaturated polymerizablemonomers selected from benzyl (meth)acrylates, a substituted orunsubstituted styrene, and a combination thereof; and (b) at least 20mol % and up to and including 35 mol % of recurring units derived from(meth)acrylic acid, all recurring unit amounts being based on the totalrecurring units in the water-soluble polymer.
 7. The aqueousparticle-free inkjet ink composition of claim 1, wherein thewater-soluble polymer is present in the aqueous particle-free inkjet inkcomposition in an amount of at least 1.5 weight % and up to andincluding 3 weight %, and the water-soluble dye colorant is present inan amount of at least 4 weight % and up to and including 8 weight %, allamounts being based on the total weight of the aqueous particle-freeinkjet ink composition.
 8. The aqueous particle-free inkjet inkcomposition of claim 1, wherein the water-soluble polymer is one or moreof Polymers 1 through 9 below: Polymer 1: poly(benzylmethacrylate-co-stearyl methacrylate-co-methacrylic acid) (35:30:35weight ratio), neutralized with potassium hydroxide; Polymer 2:poly(benzyl methacrylate-co-stearyl methacrylate-co-methacrylic acid)(35:30:35 weight ratio), neutralized with dimethylethanolamine; Polymer3: poly(styrene-co-α-methyl styrene-co-acrylic acid-co-ethoxyethoxyethylacrylate) (52:7:29:12 weight ratio), neutralized with potassiumhydroxide; Polymer 4: poly(styrene-co-methacrylic acid) (64:36 weightratio), neutralized with dimethylethanolamine; Polymer 5: poly(laurylmethacrylate-co-methacrylic acid) (67:33 weight ratio), neutralized withdimethylethanolamine; Polymer 6: poly(benzyl methacrylate-co-methacrylicacid) (67:33 weight ratio), neutralized with potassium hydroxide;Polymer 7; poly(benzyl methacrylate-co-stearylmethacrylate-co-methacrylic acid) (63.5:15:21.5 weight ratio),neutralized with potassium hydroxide; Polymer 8: poly(benzylmethacrylate-co-styrene-co-methacrylic acid) (30:45:25 weight ratio),neutralized with dimethylethanol amine; and Polymer 9: poly(benzylmethacrylate-co-methacrylic acid) (77:23 weight ratio), neutralized withpotassium hydroxide.